Three-way call detection using steganography

ABSTRACT

Three-way call detection is an important component of correctional facility telephone equipment because it helps restrict calling access by certain persons to certain restricted telephone numbers. Various three-way call detection methods exist, but all of these implementations suffer from one major limitation—the detection accuracy is not as high as is desired by the industry. The present invention improves detection accuracy, using “steganography.” Steganography involves hiding one set of data or signals within another signal or carrier in such a way that its presence is virtually imperceptible to the end recipient, and potential even the originator of the carrier signal. Television producers use steganographic methods to encode data in video signals for security, distribution monitoring, piracy-control, and other reasons. The present invention involves a device that connects to a telephone, either at the line or handset interfaces, and produces a known signal that is steganographically hidden within the user&#39;s normal voice signal. Detection equipment, residing at another location, monitors signals on telephone calls and tests for the presence of the hidden signal or data generated by the invention. Once detection is achieved, appropriate action, such as terminating or recording the call, may be taken. Improved accuracy is achieved by choosing the appropriate hidden signal(s)—ones that never occur in the course of normal conversation and never get generated by telephone company equipment. Once detected, the presence of the signal guarantees the call was made to the restricted party from a particular phone line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/910,566, filed 4 Aug. 2004, now pending, which claims the benefit ofU.S. provisional application No. 60/492,288, filed 5 Aug. 2003. Theforegoing are hereby incorporated by reference as though fully set forthherein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention relates generally to telephony, and more particularly toa method and a system for detecting when a party has been added to apre-existing telephone call.

b. Background Art

Modern phone companies offer a number of convenient services to theircustomers. One such service is called conference-calling, third-partycalling or “three-way” calling. This allows an originating caller and arecipient caller, who establish a conversation, to engage the phonesystem to dial and connect a third party into the conversation as well.

Three-way calling is a convenient feature in many situations, but posesa problem in others. Inmates in correctional facilities have the abilityto originate phone calls to others, but typically are not able to callthose numbers on a given inmate's “restricted-number” list. Persons onthe restricted list may include judges, prosecutors, other lawyers,victims of the inmate's crimes, or even other family members (especiallyin the case of protection-from-abuse (PFA) cases). By using three-waycalling, inmates are able to contact people on these restricted lists.They first call a number not on the restricted list (a friend, forexample), then have the friend perform a three-way call to therestricted number.

Although current prior art and patented devices attempt to solve thethree-way call problem, they all suffer from one major limitation;namely, they are not 100% effective at detecting such calls. Inventionssuch as this are useful in correctional-institutes telephony markets,but some percentage of inmate calls (as much as 10%, 20%, or even 30%)which use three-way calling to get another party on the line, inviolation of call-restriction rules, will get past the modern three-waycall-detection system undetected and on to the intended party.Commercial pressures exist for the accuracy rates to be higher, and ifpossible, to be 100% to handle those calls that escape detection bypresent equipment and techniques.

BRIEF SUMMARY OF THE INVENTION

Particular implementations of the present invention, which have yieldedreliable and accurate three-way call detection systems and methods, willnow be described. These present inventions use steganography to createefficiencies and improve the reliability of three-way call detection.

Disclosed herein is a three way call detection system for detecting theaddition of a third party to an pre-existing telephonic connectionbetween a first party and a second party, said system comprising an IDgenerator for generating identification data; a spread spectrummodulator for imposing the identification data onto a carrier signal,said modulator outputting a modulated identification signal havingfrequency components that are capable of being transmitted via atelephonic connection; a source telephone for participating in atelephone conversation; a signal combiner for adding the modulatedidentification signal to the telephone conversation to form an encodedtelephone signal, the modulated identification signal beingimperceptible to a human listener of the telephone conversation; aremote telephone for receiving the encoded telephone signal; and aspread spectrum demodulator for demodulating the modulatedidentification signal to extract the identification data. Theidentification data preferably is selected from the following: apersonal identification number, a user number, an inmate identificationnumber, a user name, data identifying the local telephone exchange ofthe source telephone, a telephone number of the source telephone, anddata identifying the institution where the source telephone is located.The system further may comprise a processor for extracting the modulatedidentification signal from the encoded telephone signal. The spreadspectrum modulator may impose the identification data onto a carriersignal by subdividing the identification data into a plurality ofsubdivisions and encoding the plurality of subdivisions on a pluralityof frequency bands using a predetermined sequence. The spread spectrumdemodulator optionally may demodulate the modulated identificationsignal, extract the plurality of subdivisions of the identification dataand assemble the plurality of subdivisions of the identification datainto the identification data using the predetermined sequence. Thepredetermined sequence preferably is a predetermined pseudo-random code.The system further may comprise a code generator for generating apseudo-random code, and the predetermined sequence may be apseudo-random code generated by the code generator. The signal combinerpassively may add the pseudo-random code to the telephone conversationat a predetermined plurality of frequencies.

Also disclosed herein is a method for detecting the addition of a thirdparty to an pre-existing telephonic connection between a first party anda second party, comprising generating identification data; subdividingthe identification data into a plurality of subdivisions; encoding theplurality of subdivisions on a modulated identification signal, themodulated identification signal comprising the subdivisions of theidentification data encoded on a plurality of frequency bands using thepredetermined sequence; adding the modulated identification signal to atelephone signal to form an encoded telephone signal, the modulatedidentification signal being imperceptible to a human listener of thetelephone signal; transmitting the encoded telephone signal across atelephonic connection; receiving the encoded telephone signal at aremote location; extracting the modulated identification signal from theencoded telephone signal; and demodulating the modulated identificationsignal to extract the plurality of subdivisions of the identificationdata; and assembling the plurality of subdivisions of the identificationdata to re-create the identification data using the predeterminedsequence.

Also disclosed herein is a system for identifying a source telephone,said source telephone having identification data associated therewith,the system comprising a spread spectrum modulator for encoding theidentification data into a spread spectrum signal using a spreadingcode; and a telephone interface for coupling to a telephone to add thespread spectrum signal to an output of the telephone. The system furthermay comprise a processor for coupling to a telephone to extractidentification data using a spread spectrum demodulator and a spreadingcode. The spread spectrum modulator may be a selected one of a BPSKmodulator, an MPSK modulator, and a QPSK modulator. The telephoneinterface may be coupled to a selected one of a telephone handset, aTIP/RING pair of a standard analog POTS line, and a digital telephoneinterface. The telephone interface preferably passively adds the spreadspectrum signal to a telephone signal by a selected one of transformercoupling and capacitive coupling.

Also disclosed herein is a system for decoding a modulatedidentification signal imbedded in a telephone signal wherein theidentification signal is capable of identifying a source of theidentification signal, the system comprising a receiver for receiving atelephone signal, which telephone signal comprises an audio signal and amodulated identification signal, said modulated identification signalhaving identification data embedded therein; and a spread spectrumdemodulator for demodulating the modulated identification signal toextract the identification data. The system further may comprise aprocessor, coupled to the receiver, for extracting the modulatedidentification signal from the telephone signal. The system further maycomprise a comparator for comparing the identification data to apredetermined set of restricted data; and a restricted call responsemodule for executing at least a selected one of the following responsesif the identification data matches any of the predetermined set ofrestricted data: (1) turning off a microphone of a telephone, (2)disconnecting a telephone call, (3) recording any conversation that mayoccur on a call, (4) playing a prerecorded message, (5) recording calldata, (6) disabling the speaker of a telephone, and (7) adding adisruptive signal to a telephonic connection. The system further maycomprise a code generator for generating a pseudo-random code; a spreadspectrum modulator for imposing the identification data onto a carriersignal using the pseudo-random code, said modulator outputting amodulated identification signal having frequency components that arecapable of being transmitted via a telephonic connection; and a signalcombiner for adding the modulated identification signal to an audiosignal of a telephone microphone, said combiner producing a telephonesignal as an output.

Also disclosed herein is a method for identifying a source telephonecomprising providing a data identification pattern for identifying asource telephone; generating a reference spreading code using a randomnumber generator; encoding the data identification pattern into anidentification signal using the spreading code and a spread spectrummodulator, thereby generating a modulated identification signal;combining the modulated identification signal with an audio signal of amicrophone output of a telephone to form a composite telephone signal,the modulated identification signal being imperceptible to a humanlistener of the composite telephone signal; and transmitting thecomposite telephone signal across a telephonic connection. The methodfurther may comprise receiving the composite telephone signal from asource telephone at a remote location; multiplying the compositetelephone signal by the reference spreading code to form a signalproduct; filtering or correlating the signal product to extract themodulated identification signal from the digital data; comparing themodulated identification signal with a predetermined set of restricteddata signals to determine if the modulated identification signal matchesat least one of a predetermined set of restricted data signals. Themethod further may comprise performing an envelope detection on thefiltered signal product, if needed; and comparing the output of theenvelope detection with a threshold value to determine if the signalproduct matches at least one of a predetermined set of restricted datasignals. The method further may comprise initiating a restricted callresponse if the signal product matches at least one of a predeterminedset of restricted data signals, the restricted call response beingselected from the group of following choices: (1) turning off amicrophone of a telephone, (2) disconnecting a telephone call, (3)recording a call, (4) playing a prerecorded message, (5) recording calldata, (6) disabling the speaker of a telephone, and (7) adding adisruptive signal to a telephonic connection. The step of receiving thecomposite telephone signal further may comprise providing ananalog-to-digital converter; and converting the composite telephonesignal from an analog signal to a digital signal. The method further maycomprise demodulating the modulated identification signal to extract thedata identification pattern using a reference spreading code.

Also disclosed herein is a method for identifying a source of atelephone signal received by a remote telephone comprising receiving acomposite telephone signal from a source telephone at a remote location,the composite telephone signal comprising an audio signal representativeof a telephone conversation and a modulated identification signal, themodulated identification signal having identification data that wasembedded therein using a reference spreading code; extracting themodulated identification signal; and analyzing the extracted modulatedidentification signal to determine if the identification signal containsdata that matches at least one of a predetermined set of restricted datasignals. The step of extracting the modulated identification signalfurther may comprise multiplying the composite telephone signal by thereference spreading code to form a signal product; and extracting themodulated identification signal. The method preferably comprisesinitiating a restricted call response if the signal product matches atleast one of a predetermined set of restricted data signals, therestricted call response being selected from the group of followingchoices: (1) turning off a microphone of a telephone, (2) disconnectinga telephone call, (3) recording a call, (4) playing a prerecordedmessage, (5) recording call data, (6) disabling the speaker of atelephone, and (7) adding a disruptive signal to a telephonicconnection. The step of receiving the composite telephone signal furthermay comprise providing an analog-to-digital converter; and convertingthe composite telephone signal from an analog signal to a digitalsignal. The method further may comprise demodulating the modulatedidentification signal to extract the identification data using areference spreading code.

Also disclosed herein is a three way call detection system for detectingthe addition of a third party having a third phone to an pre-existingtelephonic connection between a first party having a first phone and asecond party having a second phone comprising an ID generator forgenerating identification data; a spread spectrum modulator for imposingthe identification data onto a carrier signal, said modulator outputtingan identification signal having frequency components that are capable ofbeing transmitted via a telephonic connection; a signal combiner foradding the identification signal to an output from a microphone of thefirst phone, thereby producing a combined output signal, saididentification signal being added at a level that is imperceptible to ahuman ear that hears the combined output signal; a signal receivercoupled to a third phone, for receiving the combined output signal aftertransmission across a telephonic connection from the first phone to thethird phone; a spread spectrum demodulator for demodulating theidentification signal to extract the identification data; and aprocessor to analyze the identification data. The output from themicrophone may comprise a digital signal and the signal combiner maycomprise a selected one of a digital adder or a microcontroller thatdigitally adds the identification signal to the output from themicrophone. The output from the microphone may comprise an analog signaland the signal combiner may comprise a transformer for coupling theidentification signal to the output from the microphone. The systemfurther may comprise a digital signal processor coupled to the signalreceiver for extracting the identification signal from the combinedoutput signal. The processor preferably analyzes the identification datato determine whether the connection is authorized. The third phoneoptionally is a component of a local telephone system, and the processoroptionally analyzes the identification data and reports its analysis tothe local telephone system. The combined output signal may travel atelephonic connection between the first phone to the second phone, andtravels a telephonic connection between the second phone and the thirdphone.

Also disclosed herein is a detection system for detecting a telephonicconnection between a first phone and a second phone comprising an IDgenerator for generating identification data; a spread spectrummodulator for imposing the identification data onto a carrier signal,said modulator outputting an identification signal having frequencycomponents that are capable of being transmitted via a telephonicconnection; a signal combiner for adding the identification signal to anoutput from a microphone of a first phone, thereby producing a combinedoutput signal, said identification signal being added at a level that isimperceptible to a human ear that hears the combined output signal; asignal receiver for receiving the combined output signal aftertransmission across a telephonic connection; a spread spectrumdemodulator for demodulating the identification signal to extract theidentification data; and a processor to analyze the identification data.The system further may comprise a digital signal processor coupled tothe signal receiver for extracting the identification signal from thecombined output signal. The first phone may be a restricted phone, andthe processor may determine whether the connection between therestricted phone and the second phone is unauthorized. The processor maybe coupled to the second phone, and upon the detection of anunauthorized connection, the processor may cause one or more of thefollowing actions to occur: a) recording a conversation between therestricted phone and the second phone; b) terminating the connectionbetween the restricted phone and the second phone; c) disabling aspeaker of the second phone; d) adding an audible disruptive signal tothe telephonic connection; and e) adding an audible warning message tothe telephonic connection. The second phone may be a restricted phone,and the processor may determine whether the connection between the firstphone and the restricted phone is unauthorized. The processor may becoupled to the second phone, and upon the detection of an unauthorizedconnection, the processor may cause one or more of the following actionsto occur: a) recording a conversation between the restricted phone andthe second phone; b) terminating the connection between the restrictedphone and the second phone; c) turning off a microphone of therestricted phone; d) adding an audible disruptive signal to thetelephonic connection; and e) adding an audible warning message to thetelephonic connection. The signal receiver may be coupled to a thirdphone that is a component of a local telephone system, and the processormay analyze the identification data and report its analysis to the localtelephone system.

The present invention overcomes the problems and disadvantagesassociated with conventional systems and methods, and provides improvedsystems and methods whereby the additions of new parties to a telephonecall may be detected.

Other embodiments and advantages of the invention are set forth in partin the description that follows, and in part, will be obvious from thisdescription, or may be learned from the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a block diagram of one embodiment of the transmitterportion of the present invention. FIG. 1 shows five distinct subsystems:the data ID pattern, the random number generator, the spread spectrummodulator, the drive circuitry, and the phone interface.

FIG. 2 illustrates the operation of a BPSK Modulator of one embodimentof the present invention.

FIG. 3 represents a block diagram of one embodiment of the receiverportion of the present invention. FIG. 3 shows three distinctsubsystems: the signal interface, code acquisition & tracking, and thedata demodulator.

FIG. 4 represents an expanded block diagram of the embodiment of FIG. 3showing the elements of the code acquisition & tracking subsystem.

FIG. 5 represents a sample spectral plot of 4095 1-bit samples comingout of a 12-bit LFSR noise generator used in one embodiment of thepresent invention.

FIG. 6 represents a sample plot of a speech waveform used in testing ofone embodiment of the present invention.

FIG. 7 represents a plot of the speech samples as in FIG. 6, but withthe 12-bit LFSR noise sequence of FIG. 5 added to it.

FIG. 8 represents a sample plot of the results of correlating the 4095sample PRN sequence of FIG. 5 with the signal+noise waveform of FIG. 7and the PRN sequence itself.

FIG. 9 represents a sample plot of the results of a correlation betweenthe clean speech signal of FIG. 6 and the PRN sequence of FIG. 5.

FIGS. 10-13 illustrate four possible operating modes for the presentinvention: (1) on a typical wired telephone, attached to an analog POTSphone line as shown in FIG. 10 and FIG. 13; (2) between a telephone andits handset as shown in FIG. 11; (3) with an external microphone and acellular phone as shown in FIG. 12; and (4) attached to an analog POTSphone line with multiple wired telephones as shown in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

When an end-user listens to a signal such as a radio station, he adjustshis radio set to a number, for example 91.5, on the FM band. For the FMband, the 91.5 means 91.5 MHz, which is the frequency of the signalcarrying the music (or “carrier” frequency).

Each radio station transmits its music or other content on a separatecarrier frequency, such as 91.5 MHz, 91.7 MHz, or 96.3 MHz. The end-userselects a desired station by tuning the radio to receive a predeterminedcarrier frequency. The advantage of such a transmission system is itssimplicity, which leads to very low cost products.

The disadvantage comes when you need to send a signal between multiplepoints, say between two soldiers in the field, in such a way that no oneelse can pick up the signal. If one soldier sent the signal at onefrequency, 150 MHz, and the other soldier tuned his radio to the same150 MHz, he would receive the signal. But, so would everyone else whotuned his or her radio to 150 MHz as well. This is the situation thatspread spectrum was invented to solve.

The idea behind direct sequence spread spectrum is to take a signal thatneeds to be transmitted, to electronically take the power that normallywould lie at a fixed frequency (i.e. 150 MHz), to divide the signal intomany subdivisions, and to transmit each one at a different frequency atmuch lower power levels for each frequency. For a more detaileddiscussion of steganography, see U.S. Pat. No. 6,330,335, which patentis hereby incorporated by reference in its entirety.

The receiving device in this situation needs to know exactly how thesignal was subdivided in the first place, so it can gather all thepieces from the various frequency bands and reconstruct the signal.Since an eavesdropper needs to know: (1) where all the pieces of thesignal are placed after being subdivided; (2) exactly how to put themall back together correctly; and (3) have the technical competence togather up all the signal subdivisions across wide frequency bands, itmakes intercepting the signal(s) extremely difficult.

Since the subdivisions of the signal are all transmitted at much lowersignal levels (volume) than the original signal, they can be easilyhidden inside of another signal, such as a telephone conversation.

Any spread spectrum or other communication system consists of at leasttwo parts: a transmitter and a receiver. The receiver portion can beimplemented any number of ways; either within the phone system oroutside of it; either by hardware equipment and minimal software, or bynormal phone company hardware and lots of software.

The present invention will transmit a known set of data patterns ontothe POTS phone line of an attached telephone, using direct sequencespread spectrum (DSSS). The resulting amplitude of the DSSS signal islow enough so as to be virtually imperceptible to either the originatoror the recipient of the phone call.

As will be clear to one of ordinary skill in the art, the presentinvention has applications outside of 3-way call detection. Caller-ID,for example, identifies a phone line and not an individual phone, whichis of limited utility considering the rapid proliferation of cellphones. This invention—if embedded in cell phones—could identify thephone identity as well as (w/caller ID) the phone number being used byit.

Furthermore, it can be used to send data over the phone lines as aperson is talking, without their knowledge. For example, a modificationof the present invention could be hooked to sensors for gas and electricmeters. A consumer can simply place a call to the electric company;listen to a recording, and in the meantime, meter readings are sentright to the utility's computers, eliminating the need for meterreaders.

In a preferred embodiment of the present invention as shown in FIG. 1,the block marked “Data ID Pattern” contains digital codes that the userof the present invention wishes to pass from a phone being monitored toanother phone or system. Example codes include a serial number and ashort digital string such as “XYZ Company, Model ABC, Serial No.123456.” These digital codes are preferably unique to the particularphone line and are stored in the invention in a non-volatile memory suchas an EPROM, PROM or EEPROM. In the correctional institution context, aninmate is preferably required to enter a user ID or access code to gainaccess to the inmate telephone system. In that case, the embedded DataID Pattern will preferably uniquely identify the inmate as the source ofthe original phone call. This can be accomplished either by generatingan inmate specific code or by appending the user ID or other inmatespecific code to the phone-line-unique code. As part of deviceoperation, the patterns are sent to the “spread spectrum modulator”block to be encoded and transmitted across the phone line.

The receiver portion of the present invention preferably is installed ona restricted destination phone, such as a phone belonging to a judge,prosecutor, or victim. If an inmate-unique or a phone-line-unique DataID Pattern is identified by the present invention in a call received atone of the above restricted destination phones, this will indicate thatan inmate has placed a three-way call to that restricted phone inviolation of institutional calling restrictions, or otherwise managed tobypass the phone restrictions of the inmate telephone system.Appropriate action, such as terminating or recording the phone call, canthen be taken. An advantage of an inmate-unique identifier is that itpermits calls to particular numbers to be restricted by individualinmate and allows the inmate originator of a restricted call toimmediately be identified, and not just the originating phone linewithin the prison.

The block marked “random number generator” is an integral part of aspread spectrum system. Its purpose is to generate a random orpseudo-random number, referred to interchangeably as random numbersherein, to provide the order in which the pieces of the signal aresubdivided or modulated and sent to the spread spectrum receiver. Themore random the generator, the tougher it is to intercept, recover andreconstruct the transmitted signal at the receiver device. If thegenerator doesn't produce numbers that are very random, it will berelatively easy to intercept the transmitted signal (and data IDpattern), as well as easy to reconstruct the signal at the receivingdevice. Depending on the desired performance of the system, therandomness of the generator can be adjusted accordingly.

The block marked “spread spectrum modulator” is the method used toproduce a modulated random signal containing the Data ID Pattern fortransmission across the phone line. This invention preferably uses aBPSK modulator for its simplicity, low cost, and ease of use whentransmitting digital data. Other modulators such as MPSK or QPSK canalso be used.

The output from the BPSK modulator is applied to driver circuitry asshown in FIG. 1; such drivers also contain bandpass filtering. Themodulated output resembles random-noise, and with the appropriate driveramplitude, can be driven onto a phone line with a level that is justbarely perceptible, or even imperceptible, to most listeners.

Two possible implementations of the block marked “Phone Interface” onthe block diagram are: (1) attaching the device to the handset of atelephone, and (2) attaching the device to the TIP/RING pair of astandard analog POTS (Plain Old Telephone System) line. In bothimplementations, the circuitry preferably is designed in such a way thatthe steganographic signal (spread spectrum-modulated Data ID Patterns ordata) is passively added to the existing conversation on the line. Foranalog POTS phones, the steganographic signal preferably is transformercoupled onto the phone line. For digital phones, the digital values ofthe spread spectrum code preferably are added to the digital datarepresenting a phone call. The analog circuit preferably uses atransformer to perform the passive adding function, while the digitalcircuit preferably uses a digital adder or a microcontroller runningsoftware that performs the add operation. Generally, it is not desirablefor correctional institutions to modify the signals of conversations ofits inmates. A recording of a conversation that has been manipulated maybe excluded from use in a courtroom, and thus, for at least this reason,it is preferred that the steganographic signal is passively added to thephone lines so as to not jeopardize the integrity of the underlyingconversation. Thus, this invention preferably uses transformer- orcapacitive-coupling to passively add the steganographic signal to anyexisting conversations.

The block marked “Signal Interface” in FIG. 3 preferably consists ofdigital telecom circuitry comprising filtering, an Analog-to-Digital(A/D) converter (preferably implemented with a telecom codec), and othercontrol circuitry to output digital data representing a receivedtelephone signal. In modern, all-digital phone systems, the filteringand A/D converter just described aren't required, so the signalinterface comprises just control circuitry, as in a T1 telecominterface.

The block marked “Data Demodulator” performs the function of extractingtransmitted data from the spread spectrum signal and presenting it tothe controlling correctional telephony system and computer(s). Decisionscan be made by the controlling system, based on this received data, asto whether to continue or cancel the call, whether to start a recordingdevice or not, or to take other appropriate actions.

The block marked “Code Acquisition & Tracking” is the most important,and most difficult, part of a spread spectrum receiver to implement. Atypical Code Acquisition & Tracking function is shown in FIG. 4.

Refer back to FIG. 1 and notice the block marked Random NumberGenerator. Remember that this random number is key to spread spectrummodulation, or “spreading” of the transmitted signal across thetransmission bandwidth. The same random number sequence is required inthe receiver, and is shown in FIG. 4 marked as “Reference SpreadingCode.” The same sequence is used for both spreading and despreading.

The Reference Spreading Code function is shown with two variableparameters that affect the sequence being multiplied by the receivedsignal—namely, Frequency and Phase. The Frequency parameter affects therate of the random number generator's clock. This is required whenoscillator instabilities and drift cause the clocks on the transmitterto differ slightly from those on the receiver. The Phase parameteraffects the starting sequence loaded into the random number generator ofthe receiver or its sequence position compared to the same generator onthe transmitter.

After being multiplied by the random number sequence and bandpassfiltered, the received signal passes through an energy detector, whichtypically performs an envelope detection. The output of the envelopedetection is compared to a threshold value and a “HIT” decision is made.If the detected output is above a pre-determined (or variable)threshold, a decision is made that the correct code acquisition sequencehas been hit. Otherwise the HIT output is false.

If the code acquisition has not been hit (the HIT output is false),adjustments to the frequency and/or phase parameters may be made to helpaccelerate the likelihood of a hit occurring, or to decrease the timeneeded to detect the random number sequence phase whereby a hit occurs.

The bases for changing these parameters are well documented in theliterature, and form a number of algorithms by which code acquisitioncan be made more robust and/or faster. Some examples of potentialalgorithms include: single search algorithms; single dwell and multipledwell algorithms, and recursion-aided sequential estimation (RASE)algorithms. Each of these algorithms has its associated advantages anddisadvantages, and thus, one may be preferred over others in any givensituation depending on the characteristics of the processing circuitryavailable in the receiver, allowable cost of the receiver circuitry, andother factors.

The characteristics of the random number (RN) generator used in both thetransmitter and the receiver play a significant role in the requiredperformance of the spread spectrum receiver. RN generators are availablewhich generate very long sequences before repeating, and others areavailable with very short sequences. If a single-search code acquisitionalgorithm is used, for example, all possible phase settings for a givenRN sequence are correlated with the received signal in an attempt to geta hit. For long sequences, this can take a very long time or requiretremendous amounts of circuitry. For short sequences, the task will bemore reasonable.

Longer sequences provide more resistance to jamming, and provide lesschance of having the signal intercepted and decoded. Shorter sequencesprovide less jamming protection and less security when intercepted, butalso require less circuitry, leading to lower-cost products.

Besides simply the length of the RN sequence affecting its performance,the mathematical structure of the code affects its performance. Suchsequences as Barker Codes, Gold codes and others are well documented inthe literature.

In one preferred embodiment, a 12-tap Linear Feedback Shift Register(LFSR) is used as a pseudorandom noise (PRN) source, as is well known inthe art. A LFSR design can be selected with an arbitrary number of bits,but the more bits used, the longer the random number sequence before itrepeats and starts over again. Security systems and Government radiostypically use LFSR sequences with 40, 50, or even 60 bit LFSR circuitsso the random numbers don't repeat for hundreds of years—providing agood level of security in the radio link.

For the present invention, it is desirable for the PRN sequence torepeat to make the demodulation circuitry and software easier and morecost-effective. A 12-bit LFSR circuit generates a pseudorandom patternthat repeats every 4095 clock cycles. For example, with an 8 kHz clockrate, the pattern will repeat approximately every.+−.2 second, allowinga 3-way call detection system to detect the call more than once everysecond.

FIG. 5 shows a spectral plot of the 4095 1-bit samples coming out of the12-bit LFSR noise generator. The spectral shape is substantially flat.This gives the PRN signal a very good “white-noise” sound when added toa telephone conversation. This signal is almost undetectable when theamplitude levels are set correctly and added to phone conversations.Typical POTS phone systems have an approximately 40-45 dB dynamic range.

In summary, the spread spectrum receiver used with the present inventioncan be a standard DSSS receiver that implements any number of algorithmsfor its code-acquisition and tracking functions. The requireddemodulator is preferably a BPSK type, and the input signal ispreferably digital, having been processed by telephony circuitry andpassed through telephony codecs.

A number of tests were conducted to validate the effectiveness of oneembodiment of the present invention. The test system was equipped withhandset and phone line interfaces and utilized a microcontrollerprogrammed with detection software that is designed to run continuouslyand output a digital bit (one or zero) every 125 μsec (which equates toan 8 kHz output rate). Each digital bit is output as the result of aPseudo-Random Noise Generator (“PRN generator”), implemented insoftware. The PRN consisted of a single data register (implemented as asoftware variable) with feedback. The timing of the software isimportant, as the calculations on the PRN register must be completed intime for a valid bit to be output at the desired interval, which was 125μsec for the test embodiment. Timing can be verified by using anoscilloscope to monitor a debug pin on the microcontroller, which istoggled every 125 μsec. The microcontroller used for the foregoing testshad internal RAM for variables, FLASH for storing the program, I/O pinsfor toggling the required signals, and an internal timer for measuringthe 125 μsec period. MATLAB was used for the DSP and signal processingportion of the system design. Segments of speech from the industrystandard “TIMIT” speech database were used for testing. TIMIT is acollection of carefully chosen speech utterances that have specialphonetic qualities that stress speech recognition systems and is widelyused for speech research. The database comprises 10 spoken sentences foreach of 420 speakers, some male and some female. FIG. 6 shows a plot ofthe speech waveform used in this testing. FIG. 7 shows a plot of thespeech samples as in FIG. 6, but with the 12-bit LFSR noise sequenceadded to it.

FIG. 8 shows the results of correlating the 4095 sample PRN sequencewith the signal+noise waveform and the PRN sequence itself. Each of thevertical “spikes” in FIG. 11 represents a point in time (during exactlyONE sample period of the 8 kHz clock) where the PRN noise sequence ofthe present invention and the noise in the signal are in perfect phase,causing the correlation output to jump up to a maximum value. Each ofthe spikes occur 4095 samples apart, because that is the repetitioninterval of the LFSR noise generator in the tested embodiment. Thedashed line on FIG. 8 is an example threshold that is preferably set inthe system software portion of the demodulator to indicate when athree-way call has been detected.

FIG. 9 shows the results of a correlation between the clean speechsignal and the PRN sequence. FIG. 9 maintains the same relative scalingas FIG. 8. Because the speech waveforms are not well correlated to thePRN noise sequence, the correlation peaks of FIG. 9 have a significantlylower magnitude than those of FIG. 8. In the absence of the PRN signal,as would be the case when there is no three-way call taking place, eventhe correlation peaks do not exceed the detection threshold.

FIG. 10 shows the invention used with a typical POTS (plain oldtelephone system) analog telephone. Telephone 1 is connected to theincoming phone line 4 by means of a standard 2-wire or 4-wire phonecable 2. Invention 3 is also connected to the incoming phone line bymeans of a similar cable. Circuitry within invention 3 allows it togenerate a signal that also gets driven onto the phone line 4 along withthe signal from the telephone 1. The scenario shown in FIG. 10 can alsobe used with cordless (but not wireless cellular) phones. Attachinginvention 3 to the phone line 2 can be done with a cordless phone 1 aswell as with a standard old-style (handset attached to the base)telephone.

An alternative scenario is shown in FIG. 11 where the invention 5 isconnected between the phone 7 and its handset 6. This scenario can beused for single-phone installations where only a single phone needs theinvention and not all the phones on a POTS line as shown in FIG. 11.This scenario also has advantages in physical size and not requiring FCCcompliance, which can help reduce its development and production costs.

The invention can also be used with wireless (cellular) phones, as shownin FIG. 12. Invention 1 is attached to a microphone 9 by means of amicrophone cable 10, as well as to the cell phone 13 by means of anothercable 12. Invention 11 receives the signal from the microphone 9 as theuser speaks, adds the steganographic signal to the microphone signal,and then drives the resulting signal into cell phone 13 fortransmission.

FIG. 13 shows a scenario similar to FIG. 11, but for a multiple-phoneinstallation such as in a home or business. Invention 16 generates thesteganographic signal and drives it onto the phone line 15 along withthe signal from one or more phones 14 for transfer to the POTS line 17.

Other embodiments and uses of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. The specification and examples shouldbe considered exemplary only with the true scope and spirit of theinvention indicated by the following claims. As will be understood bythose of ordinary skill in the art, variations and modifications of eachof the disclosed embodiments can be easily made within the scope of thisinvention as defined by the following claims.

1. A method for detecting the addition of a third party to apre-existing telephonic connection between a first party and a secondparty, said method comprising: generating identification data;subdividing the identification data into a plurality of subdivisions;encoding the plurality of subdivisions onto a carrier signal to create aspread spectrum modulated identification signal, the spread spectrummodulated identification signal comprising the subdivisions of theidentification data encoded on a plurality of frequency bands using apredetermined sequence; adding the spread spectrum modulatedidentification signal to a telephone signal to form an encoded telephonesignal, the spread spectrum modulated identification signal beingimperceptible to a human listener of the telephone signal; transmittingthe encoded telephone signal across a telephonic connection; receivingthe encoded telephone signal at a remote location; extracting theplurality of subdivisions of the identification data from the spreadspectrum modulated identification signal; and assembling the pluralityof subdivisions of the identification data to re-create theidentification data using the predetermined sequence.
 2. The methodaccording to claim 1, further comprising comparing assembledidentification data to a predetermined set of restricted data todetermine if a third party has been added to the pre-existing telephonicconnection.
 3. The method according to claim 1, wherein the step ofgenerating identification data comprises inputting at least onealpha-numeric identifier of the first party.
 4. The method according toclaim 3, wherein the at least one alpha-numeric identifier of the firstparty is selected from the group consisting of a personal identificationnumber, a user number, an inmate identification number, a user name, alocal telephone exchange, a source telephone number, and any combinationthereof.
 5. The method according to claim 1, wherein the predeterminedsequence is a predetermined pseudo-random code.
 6. The method accordingto claim 1, wherein the step of adding the spread spectrum modulatedidentification signal to a telephone signal comprises passively addingthe spread spectrum modulated identification signal to the telephonesignal.
 7. The method according to claim 2, further comprisinginitiating a response upon detecting the addition of a third party tothe pre-existing telephonic connection between the first party and thesecond party.
 8. The method according to claim 7, wherein the responseis selected from the group consisting of: turning off a microphone of atelephone; disconnecting the telephonic connection; recording a call;playing a pre-recorded message; recording call data; disabling a speakerof a telephone; adding a disruptive signal to the telephonic connection;and any combinations thereof.
 9. A method of monitoring use of a sourcetelephone by a caller, the method comprising: generating identificationdata identifying at least one of the caller and the source telephone;encoding the identification data into a modulated identification signalusing a spread spectrum modulator; combining the modulatedidentification signal with an audio output signal of the sourcetelephone to form a composite telephone signal; transmitting thecomposite telephone signal from the source telephone to a destinationtelephone; receiving the composite telephone signal at the destinationtelephone; extracting he identification data from the compositetelephone signal using at least a spread spectrum demodulator; andanalyzing the extracted identification data to determine if theextracted identification data contains data that matches at least one ofa predetermined set of restricted data.
 10. The method according toclaim 9, further comprising initiating a restricted call response upondetermining that the extracted identification data contains data thatmatches at least one of a predetermined set of restricted data.
 11. Asystem for detecting unauthorized use of a source telephone to contact adestination telephone by a caller having restricted calling privileges,the system comprising: a spread spectrum modulator that encodes anidentification signal to generate an modulated identification signal,the identification signal including data identifying at least one of thesource telephone and the caller; a signal combiner coupled to an outputof the spread spectrum modulator and the source telephone that combinesthe modulated identification signal and an output signal of the sourcetelephone into a composite telephone signal; a signal receiver coupledto the destination telephone that receives the composite telephonesignal and extracts therefrom the modulated identification signal; aspread spectrum demodulator coupled to the signal receiver that decodesthe identification signal and extracts therefrom the data identifying atleast one of the source telephone and the caller; and a comparatorcoupled to the spread spectrum demodulator that compares the dataidentifying at least one of the source telephone and the caller to apredetermined set of restricted data.
 12. The system according to claim11, further comprising an unauthorized call response processor thatinitiates an unauthorized call response when the data identifying atleast one of the source telephone and the caller matches at least one ofthe predetermined set of restricted data.